I had the same issue after flashing. I got it fixed with the following steps:
1 Download the app "GPS Status and Toolbox" in the play store.
2 Go to the menu in "Manage A-GPS data" select "reset" then "download" in the same window
3 Go back to the main window and keep the phone busy outdoors for approximately 5 min. and wait for the GPS lock. (You will see it in the app)
4 That's it. Since then my GPS is back and stable as it was in stock.